Electron discharge tube



Sept. 13, 1938. M. KNOLL 2,130,280

ELECTRON DISCHARGE TUBE Original Filed April 6, 1934 66 Q Q INVENTOR A xV BY 7 ATTORNE Patented Sept. 13, 1938 "UNITED STATES PATENT OFFICEELECTRON DISCHARGE TUBE Germany Application April 6, 1934, Serial No.'719,287.

newed July'3, 1937. 1933 12 Claims.

The. present invention is concerned with discharge tubes comprising acathode, an anode and one or more interposed grid electrodes, and -moreparticularly with the construction and disposition of said electrodes insuch tubes. The stream or current of electrons in discharge tubes hasheretofore been conceived of as being a stream of a continuous medium(the so-called electron gas" or cloud) proceeding from the cathode orfilament through the control electrode to the anode or. plate. However,more recent investigations have demonstrated that a current of electronswhich propagates throimh a broken or apertured electrode such as a grid,is resolved, under certain circumstances, into discrete con- .stituentor individual pencils or brushes of rays sharply separated from oneanother which, in the presence of sufficiently high speeds and sum-:ciently low currentdensities, exhibit a behavior satisfying the laws ofgeometrical electron optics. If an electron having a velocity of Uhenters betweentwo equipotential surfaces of potential Uo of thenon-homogeneous field at the opening of an apertured or non-continuouselectrode, it ",wil1'be refracted or deflected, according" to the signof the electrode, in the direction of, or away from, the perpendicular.For the space into which the electron will get after having passedthrough the two equipotential surfaces, there will then hold thefollowing index of refraction:

, On the basis of this law of refraction of elec- 35 tron-optics, it-ispossible to trace, both graphically and bywcalculation,.the differentelectron paths or trajectories on passing through theequipotentialsurfaces in the openings of apertured .-or non-continuouselectrodes, in analogy withthe behavior of a luminous ray inside ameexhibiting non-homogeneous refractiveness.

---Whatis important and essential in connection with-the inventionhereinafter to be described is a proper appreciation of this fact thatthe laws of' electron-optics hold good not only, as is well known, forhigh electron velocities and comparativelysmall currents, but also forrelatively lowvoltagesand. relatively large currents such as occur inordinary amplifiers, in spite of electrostatic repulsion of theelectrons between one another. If the electrode systems there used areinvestigated on the basis of the electron-optical viewpoint ratherthanthe hydrodynamic one, as has heretofore been done,

In Germany April 12,

it will be discovered that the electrons, at the openings of the variousapertured electrodes, have to deal with or are subject to approximatelycylindrical or calotte-shaped (hemispherical) equipotential surfaceswhich come to act upon them in the way of small lens systems, and theresult is that, posteriorly of each opening of an apertured electrode,there arises a tiny easily definable pencil or brush of rays, thecross-section and shape of which is a function of the form and the sizeof the said opening.

The object of the present invention, by adopting suitable dimensions forthe openings of the apertured electrodes, by the disposition of theelectrodes themselves, by the development of their surface, and by themutual or relative positions of the openings of the various electrodes,is to act upon or influence the characteristic of the discharge tube incertain or prearranged ways. By the formation of electric lenses ashereinbefore mentioned, inside the openings of apertured electrodes-fociwill be set up posteriorly thereof in which the electrons will befocused. Itis possible to cause the foci to register with the openingsof the next apertured electrode or electrodes so that the passage ofelectrons thereto (or absorption of electrons by the same) will beavoided. This means a great relief for these electrodes, and this isespecially advantageous whenever auxiliary electrodes maintained at ahigh positive potential are involved, as is true, e. g., of screen-gridsand space-charge grids. Moreover, owing to the fact that the electronsmay be'exactly concentrated or focused in the openings of a controlelectrode, it is feasible to secure increased controlling power oraccuracy and greater slope (mutual conductance) of the tube. Byintentional shifting of the foci away from the center of the opening ofthe following apertured electrode it is further feasible to flatten thecharacteristic, indeed, to impart to the latter any kind of shape ortrend, for instance, to obtain also aform following an exponential orlogarithmic law. If, furthermore, the cathode surface is given a shapewhich agrees or conforms with that of the equipotential surfacesoccasioned by an apertured electrode, this affords a chance, especiallyin the presence of small inter-grid distances, to make the load of thecathode more uniform, to derive or extract from the latter a higheremission, at the same control voltage, and thus to secure a furtherincrease in the slope of the tube.

For a better understanding of the basic idea underlying this invention,the same will be explained by reference to the accompanying drawing.

In Fig. lis shown a schematic longitudinal section through the electrodesystem of a tetrode tube which comprises a cathode K, a spacecharge gridGs, a control grid Go, and an anode A. Both grid electrodes, forinstance, shall be assumed to be of the mesh or gauze metal type.Inasmuch as all parts of the same electrode are at one and the samepotential, there arises a field distribution such as indicated in thegraphs, the fine solid lines indicating the form of the equipotentialsurfaces. It can be readily seen that the potential surfaces between twogrid wires resemble the shape of a biconcave lens, and the vicinity ofan individual grid wire the form of a biconvex lens. Whether theseequipotential surfaces exercise one action or the other upon theelectrodes, in other words, whether they focus or disperse, will dependupon the sign of the potential gradient in the potential surface inquestion, seeing that the electrons in the one instance are attracted,and are repelled in the other. In view of the positive potential of thespace-charge grid, the electrons emanating from the cathode areconcentrated in pencils of rays B which are located exactly posteriorlyof the wires of the space-charge grid. According to pre-supposition, thecontrol grid is at a negative potential, and it is for this reason thatin this case, the interstitial spaces between its wires act like anotherfocussing lens, and focus the electrons upon the wire strips of theanode A. It can be readily understood that the constituent ray pencilswill penetrate through the control grid without an incidental impedimentof their trajectories if the openings of each grid are exactly alike.I'his stipulation is fulfillable in various ways, for instance, by thatholes of like shape are punched in a lamination or sheet, said holesbeing equally spaced apart. The openings could have also the form ofslits running parallel to the cathode. Also the so-called mesh gridsmade of wire gauze satisfy the said condition, at least approximatelyso, through in that case it is advisable to roll the gauze down orflatten it in order that it may present throughout the same thickness orgauge; for the requirement of having uniform thickness in aperturedelectrodes in sofar as they are traversed by the stream of electrons, isessential for the uniform formation of the ray pencil. t is for thisreason that grids attached to stays constructed according toconventional methods would appear unsatisfactory, While spiral grids,fundamentally speaking, are admissible, for they may be conceived ashelically wound cylinder lenses, it must be borne in mind that thisuniform condition should not be disturbed by stays or similar supportingmeans. From the distribution of the electrons as illustrated, it can beseen that when making the anode or plate from a wire gauze or netting ofdefinite dimensions, favorable thermal radiation is feasible, on the onehand, while, on the other hand, provided that the pencil of rays impingeexactly upon the anode surface, no electrons will be able to reach thespace in the rear of the anode where they are liable to give rise to allkinds of trouble.

Fig. 2 shows a cross-section of a tetrode tube comprising a cathode K, acontrol grid Go and an anode A. In addition an auxiliary electrode Go.is mounted between the control grid and the cathode which, for thereasons hereinafter to be discussed in more detail, is so designed andarranged that the openings thereof come to register or coincide exactlywith the openings of the control grid. The auxiliary grid is maintainedat the same or at a negative potential in relation to the cathode. As aconsequence, the spaces between the grid wires act as condenser lensesand they thus focus and concentrate the electrons in the intersticesbetween the wires of the control grid. Hence, the electrons find nochance to impinge upon the control grid proper. This circumstance wouldseem important especially when the tube is operated inside the range ofpositive grid voltages, inasmuch as then no grid current will be able toarise, and since the control potential source is relieved of load. As anatural result, as will be noted, such distortions as will arisenormally when working within the positive grid potential region, willhere be avoided.

This construction is of utmost importance particularly also inconnection with transmitter valves seeing that in the case of these, onthe one hand, owing to higher operating voltages, far larger gridcurrents will arise, while, on the other hand, due to the largedimensions of the electrode system, the chances for insuring precisemounting and assembling in conformity with the demand hereinbeforeindicated, that is, correct registering of grid openings, are still moreeasily fulfilled.

The use of an additional auxiliary grid in a space-charge grid type oftube is illustrated in Fig. 3. The electrode system comprises thecathode K, the auxiliary Ga, the space-charge grid Gs, the control gridGo, and the anode A. What must be borne in mind is that conditionsshould be such that the openings of the space-charge grid and theauxiliary grid will exactly come to register, wheras the wires of thecontrol grid will coincide correctly with the middle of the saidopening. The auxiliary grid again is maintained at the same or at anegative voltage in reference to the cathode, whereas the space-chargegrid, as usual, is connected with a positive biasing potential. In thismanner conditions, on the one hand, are made such that the space-chargegrid will not be struck by electrons, as is true of Fig. 1. As a resultthe appreciable consumption of energy that has sofar been inseparablefrom the use of a space-charge grid and which has proved a hindrance tothe wider introduction of this type of tube, is obviated. On the otherhand, the electrons through the space-charge grid can be focussed in theopenings of the control grid, and they are there subjected to a maximumcontrol action.

Fig. 4 represents a cross-section through the electrode system of ascreen-grid tube containing the cathode K, the control grid Go, thescreen grid Gsc, and the anode A. The two grids may each consist, forexample, of a sheet-metal cylinder in which are punched slots runningparallel to the cathode. The two apertured electrodes are so disposed inreference to each other that the openings will come to registerprecisely. The potential of the control grid should always be negativein reference to the cathode. In this manner, the openings of the controlgrid act like cylindric condenser lenses, and as a result they cause theelectrons to become concentrated in the openings of the screen grid withthe conse quence that the screen grid can be practically renderedcurrentless. The practical result and success of this step manifestsitself not merely in a saving of current by the suppression of currentin the screen grid, but also in the circumstance 75 that the tubesbecome uniform, contradlstinct from-what has heretofore'been'thecase-where it was largely a question of chance'whether the unwhesof'the-screen grid'would collect orabsorb electrons or not, 'wlththeresult: that rather great disparities were observed inthe size'of thecurrents flowing in the screen grid. Variations in this regard becameparticularly annoying whenever the screen-grid voltage was tapped from.a voltage divider or potentiometer.

A further embodiment of the basic idea of this invention is indicated inFig. 5. It is well known that in the presence of a very small distancebetween control grid and cathode, the electrons will not always bederived in a uniform way along the entire surface of the cathode,indeed, that some portions of the cathode become subject to marked loadsand that these local areas furnish practically the whole electronemission, whereas neighboring portions would give off no electrons. Thiscircumstance proves unfavorable from the viewpoint of life of thecathode, and it imposes a limitation so far as the slope (mutualconductance) is concerned in the presence of intergrid distances fallingbelow the size of grip openings. Now, another object of this inventionis a novel formation of the cathode surface such that it will match andadhere to the equipotential surfaces produced by virtue of the gridconstruction.

Fig. is a longitudinal section through the electrode system of a triodetube containing the cathode K, the control grid Go and the anode A. Thetrend or shape of the potential surfaces is indicated by the horizontalsolid lines. The cathode surface has ridges or depressions the shape ofwhich is a function of the control electrode. If the latter, forinstance, consists of a mesh or gauze or of a punched perforated sheetor lamination, the said depressions are made hemispheric(calotte-shaped) so that to each grid mesh there corresponds adepression which comes to be positioned exactly in the rear of the gridopening. In the case of rodlet-type grids, corrugations will, inanalogy, be arranged parallel to the axis of the cathode, as shown inFig. 6, while with spirally-wound grids there should be provided ahelical groove having the same pitch as the grid.

Instead of providing the cathode surface with depressions, roughly theidentical effect is attainable by making the cathode bl-partite, inother words, a cylindrical electron-emission surface of the kindheretofore customary, and further, at close proximity in front thereofand conductively connected therewith, a grid whose openings come toregister with those of the control electrode.

What I claim is:

1. An electron discharge tube comprising a cathode, an anode, a gridelectrode interposed between cathode and anode, and means forcontrolling the electrostatic field in the vicinity of the gridelectrode whereby the electron flow therethrough will not be impeded,said means comprising arcuate electron emitting surfaces formed on thesurface of the cathode, which arcuate surfaces are arranged in registrywith the apertures of the grid electrode.

2. An electron discharge tube comprising an equipotential cathode havinga plurality of depressions uniformly distributed along its length, ahelical grid electrode surrounding the cathode and having its spacesbetween successive turns aligned with the cathode depressions, and anadditional electrode surrounding the grid electrode.

3. *An electron discharge tube: comprising an indirectly heated'cathodehaving a pluralityof uniformly spaced similarly-shaped electron emittingsurfaces, a perforated electrode positioned adjacent said cathode withits perforations in registry with the said-similarlyshaped cathodeemitting surfaces, and 'a'n'additional electrode surrounding theperforatedelectrode.

4. An' electron discharge tubercomprising 8. cylindrical equi-potentialcathode having formed on its surface a helically-grooved electronemitting surface of uniform pitch, a similarly pitched helically-woundgrid electrode surrounding the cathode so that the spaces betweensuccessive grid turns register with the grooved emitting surface, and anadditional electrode surrounding the cathode and the grid electrode.

5. An electron discharge tube comprising an equipotential cathode havinga plurality of depressions distributed along its length, a grid electrode surrounding the cathode and having its spaces aligned with thecathode depressions, and an additional electrode surrounding the gridelectrode.

6. An electron discharge tube comprising a cathode, an anode, a gridelectrode interposed betwen cathode and anode, and means for controllingthe electrostatic field in the vicinity of the grid electrode wherebythe electron flow therethrough will not be impeded, said meanscomprising arcuate electron emitting surfaces formed on the surface ofthe cathode and extending in directions parallel to the cathode axis,the grid electrode being in the form of rods which are also arranged inparallel relation to the cathode axis.

7. An electron discharge tube according to the preceding claim whereinthe spacings between the grid rods are in registry with the arcuatecathode surfaces.

8. An electron discharge tube comprising an equipotential cathode havinga plurality of longitudinally extending depressions uniformlydistributed around its surface, a grid electrode provided withlongitudinally extending conductors surrounding the cathode and havingthe spaces between successive grid conductors aligned with the cathodedepressions, and an additional electrode surrounding the grid electrode.

9. An electron discharge tube comprising a cathode, an anode, a gridelectrode interposed between cathode and anode, and means forcontrolling the electrostatic field in the vicinity of the gridelectrode whereby the electron flow there through will not be impeded,said means comprising spaced electron emitting surfaces in the form ofnarrow elongated strips formed on the surface of the cathode andextending in directions paral lel to the cathode axis, the gridelectrode being in the form of rods which are also arranged in parallelrelation to the cathode axis.

10. An electron discharge tube comprising an equipotential cathodehaving a plurality of longitudinally extending electron emitting stripsuniformly distributed around its surface, a grid electrode provided withlongitudinally extending conductors surrounding the cathode and havingthe spaces between successive grid conductors substantially aligned withthe electron emitting strips, and an additional electrode surroundingthe grid electrode.

11. An electron discharge tube comprising an equipotential cathodehaving a plurality of longitudinally extending depressions which areoxide coated and uniformly distributed around its surface, a gridelectrode provided with longitudinally extending conductors surroundingthe cathode and having the spaces between successive grid conductorssubstantially aligned with the oxide coated cathode depressions, and anadditional electrode surrounding the grid electrode.

12. An electron discharge tube comprising an equipotential cathodehaving a plurality of longitudinally extending oxide coated stripsuniformly distributed around its surface, a grid electrode provided withlongitudinally extending conductors surrounding the cathode and havingthe spaces between successive grid conductors substantially aligned withthe cathode strips, and an additional electrode surrounding the gridelectrode.

MAX KNOLL.

